Observations of water with Herschel/HIFI toward the high-mass protostar AFGL 2591

Abstract

Context. Water is an important chemical species in the process of star formation, and a sensitive tracer of physical conditions in star-forming regions because of its rich line spectrum and large abundance variations between hot and cold regions. Aims. We use spectrally resolved observations of rotational lines of H2O and its isotopologs to constrain the physical conditions of the water emitting region toward the high-mass protostar AFGL 2591. Methods. Herschel/HIFI spectra from 552 up to 1669 GHz show emission and absorption in 14 lines of H 2 O, H218O, and H217O. We decompose the line profiles into contributions from the protostellar envelope, the bipolar outflow, and a foreground cloud. We use analytical estimates and rotation diagrams to estimate excitation temperatures and column densities of H2O in these components. Furthermore, we use the non-local thermodynamic equilibrium (LTE) radiative transfer code RADEX to estimate the temperature and volume density of the H2O emitting gas. Results. Assuming LTE, we estimate an excitation temperature of ~42 K and a column density of ~2 × 1014 cm-2 for the envelope and ~45 K and 4 × 1013 cm-2 for the outflow, in beams of 4″ and 30″, respectively. Non-LTE models indicate a kinetic temperature of ~60-230 K and a volume density of 7 × 106-108 cm-3 for the envelope, and a kinetic temperature of ~70-90 K and a gas density of ~107-108 cm-3 for the outflow. The ortho/para ratio of the narrow cold foreground absorption is lower than three (~1.9 ± 0.4), suggesting a low temperature. In contrast, the ortho/para ratio seen in absorption by the outflow is about 3.5 ± 1.0, as expected for warm gas. Conclusions. The water abundance in the outer envelope of AFGL 2591 is ~10-9 for a source size of 4″, similar to the low values found for other high-mass and low-mass protostars, suggesting that this abundance is constant during the embedded phase of high-mass star formation. The water abundance in the outflow is ~10-10 for a source size of 30″, which is ~10× lower than in the envelope and in the outflows of high-mass and low-mass protostars. Since beam size effects can only increase this estimate by a factor of 2, we suggest that the water in the AFGL 2591 outflow is affected by dissociating UV radiation as a result of the low extinction in the outflow lobe.